1. Field of Invention
The present invention relates to a system for measuring parameters relating to fluid flow in a channel, and in particular to a system which provides an accurate measurement of average fluid flow rate which takes into account the effect of drawdown.
Flow of material through a partially filled channel or pipe is expressed by the equation EQU Q=A*V,
where Q represents the rate of flowable material through the channel; V represents the average velocity of material in the channel; and A represents the cross-sectional area of material in the channel, or wetted cross-sectional area. For a channel or pipe having a substantially circular cross-section, the wetted cross-sectional area may be expressed as EQU A=R.sup.2 * cos.sup.-1 ((R-L)/R)-(R-L) * (2*R*L-L.sup.2).sup.5,
where R represents the actual channel or pipe radius and L represents the depth of the flowable material in the channel (FIG. 3). It is noted that similar equations exist for calculating the wetted cross-sectional area of channels having different cross-sectional shapes. As a result, an accurate measurement of the depth of flowable material in the channel is essential in determining flow through a partially filled channel.
Pressure sensitive devices exist which, when placed in a fluid channel, determine fluid depth by measuring the pressure exerted on the device due to the fluid overhead. Once the fluid pressure is measured, fluid depth L may be calculated as EQU L=27.681*P,
where P represents the measured pressure acting on the probe, in psi.
Pressure sensitive devices which measure fluid depth in a channel are often placed in a stilling well--a calm, isolated area which is adjacent a stream of flowing fluid and which has a fluid level which is substantially the same as the fluid level in the stream. In some instances, however, the pressure sensitive devices are placed directly into the stream of flowing fluid, in which case their fluid depth measurements are affected by a phenomenon known as drawdown.
Drawdown is caused by the presence of objects in the stream of fluid flow. Specifically, drawdown is an effect that occurs in nonhydrostatic conditions in which the pressure exerted on the probe by the flowing fluid is actually less than the ambient pressure in the stream due to the streamlines of flow being disturbed by the probe. If the presence of the probe causes any disturbance in the streamlines of flow (FIG. 4), drawdown will occur regardless of the streamlined nature of the probe. With the pressure sensitive probe measuring fluid pressure that is less than the actual ambient pressure in the stream of fluid, fluid depth in the channel is under-represented, thereby leading to an inaccurate flow rate computation. The present invention is directed at substantially eliminating the inaccuracies associated with fluid depth measurements by taking into consideration the effects of drawdown.
2. Description of the Relevant Art
There are known probes which measure fluid flow. For example, U.S. Pat. No. 5,506,791 discloses a multi-functional device having a pressure sensitive probe for measuring fluid depth in a channel.
The above-identified reference, however, fails to disclose or otherwise suggest a system for measuring fluid velocity in a channel which takes into account the effects of drawdown.